A detail from The Cure of Folly, relegated from a Bosch to ‘workshop or follower of Hieronymus Bosch’.
Photograph: Alamy

Art museums like to present their best treasure in an authoritative way. Clearly printed labels on paintings tell you the artist’s name and when he or she lived, as well as the likely date of the work. But how reliable is that information? And if the facts change, do labels change too?

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The Prado has a problem. The Bosch Research and Conservation Project, whose findings underpin the great exhibition of this Netherlandish visionary now at the Noordbrabants Museum in Den Bosch, has declared two of the paintings in the Spanish museum’s unrivalled collection of Hieronymus Bosch not his pure handiwork after all. The Cure of Folly is now merely “Workshop or follower of Hieronymus Bosch (c1510-20)”, say researchers, while The Temptation of Saint Anthony has been relegated to “Follower of Hieronymus Bosch (c1530-40 and after 1574)”. This is how they are labelled in the exhibition catalogue.

Madrid is none too pleased. The Prado has reneged on its offer to lend these works for the show because, it says, the Dutch museum’s plan to display them as “not by Bosch” breaks an agreement and “also suggests the Museo del Prado accepts and authorises the proposed attributions”. In other words, it does not accept the de-Bosching.

When I saw these two paintings in the Prado last year, they were clearly labelled as the work of old Hieronymus himself – El Bosco in Spanish – and presumably that’s the way they’ll stay. Perhaps the surprise is not that the Prado has objected but that so many museums have cooperated to make the Den Bosch show a triumph even where this means accepting their cherished Bosches are just by a “follower”, after all.

The Bosch Research and Conservation Project has recently attributed the painter’s Temptation of St Anthony to a ‘follower’. Photograph: Alamy

We have been here before. The Bosch Research and Conservation Project, in its insistence that it can define a clear, objective and definitive corpus of the master’s works, is emulating the Rembrandt Research Project, which, since 1968, has pioneered a new exactitude in identifying the unique styles of artists. These Rembrandt researchers have spent decades dictating what is and what is not a Rembrandt. Today, many museums that once proudly displayed a Rembrandt now only have a “school of”, “studio of” or “follower of” Rembrandt.

Behind these state-of-the-art Dutch research projects lies a 200-year history of the gradual application of scientific scholarship to art. When public museums got going in the Romantic age, they displayed paintings about which very little was known. Gradually, over the course of the 19th century – especially after the invention of photography made it possible to compare good reproductions – the styles of artists were defined and established. Lots of famous names inscribed on picture frames turned out to be fantastical.

As recently as the 1890s, a painting of Medusa by Leonardo da Vinci was one of the highlights of the Uffizi gallery, in Florence. Today, it is known to be – and easy to recognise as – a baroque picture done almost a century after his death. Meanwhile, the Venetian artist Giorgione, who was once seen as much more important than Titian, has had almost all his works taken away from him. It will be fascinating to see if the Royal Academy’s forthcoming Giorgione show can restore his reputation.

This has got everything to do with big names. Caravaggio’s name sells, for instance. It draws people to galleries. No wonder the Prado insists that its single painting by Caravaggio is the real thing. It publicises its picture of David as by Caravaggio – it admits to no ambiguity. Yet, in truth, many doubt it is one of his paintings. It looks to me like a much lesser work by one of his many imitators.

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I don’t want to pick on the Prado, which is surprisingly ready to unmask its own works. The most shocking relegation of recent years came when the Prado decided one of its Goya masterpieces was not by him at all. The Colossus, a nightmarish vision of war, is really by one of Goya’s assistants, the museum announced in 2009. So, it’s not that museums won’t accept unpleasant facts about their collections. It’s just that they don’t like being told what to think by outsiders.

Museums such as the Prado and London’s National Gallery have their own laboratories and do their own research. As a National Gallery insider once told me: “We know the fakes in our collection.” Which all goes to show those labels on the wall are not monoliths of unquestioned fact. They conceal scholarly rivalries and institutional self-promotion. Perhaps it was better, after all, in more romantic times, when paintings were just shown in golden frames with “Leonardo” or “Giorgone” emblazoned below. The truth was anybody’s guess. Perhaps it still is.